1. Field of the Invention
The present invention relates to a chemical vapor deposition (CVD) apparatus used in the manufacturing of a semiconductor memory device. More particularly, the present invention relates to the gas diffusing nozzle of the apparatus from which gas diffuses so as to become deposited on the surfaces of wafers loaded in the apparatus.
2. Description of the Related Art
In general, in the process of fabricating a semiconductor device, a layer to be used as a dielectric or conductive material of the device is formed on the surface of a wafer by diffusing a gaseous chemical (vapor) onto the wafer, thereby facilitating a chemical reaction in which the layer is formed. The chemical vapor deposition processes available for forming such a dielectric or conductive layer can be classified as air pressure chemical vapor deposition, low pressure chemical vapor deposition or high pressure chemical vapor deposition.
In particular, low pressure chemical vapor deposition occurs at a pressure of 0.1 to 100 Torr. In addition to pressure, the temperature, the wafer gap, the quantity of the diffusing gas, and the like, are factors which affect the uniformity of the resultant layer of each wafer.
Chemical vapor deposition is carried out in a CVD apparatus comprising a quartz tube within which a plurality of wafers are loaded. The apparatus also includes a plurality of gas diffusing nozzles through which the process gas is supplied, and a discharge outlet through which gas in the tube is discharged from the tube.
FIG. 1 illustrates a conventional chemical vapor deposition apparatus. The apparatus includes an external quartz tube 10 and an internal quartz tube 11 disposed inside of and spaced from the external tube 10 so that a predetermined gap is provided therebetween. The top of the internal tube 11 is open. A wafer boat 12, defining slots which accommodate a plurality of wafers, respectively, is provided within the internal tube 11.
The CVD apparatus also includes a plurality of gas diffusing nozzles 13, 14, 15 leading into the space in which the wafers are positioned from a location underneath the external tube 10. The gas diffusing nozzles 13, 14, 15 are of different heights within the internal tube 11 so that the gas supplied through the nozzles will be distributed evenly over the wafers situated in the wafer boat 12.
Furthermore, the discharge outlet 16 is located across from the gas diffusing nozzles 13, 14, 15 at the other side of the external tube 10. A bell jar 17 covers the external tube 10 and is provided with a heater.
However, the prior art of the chemical vapor deposition apparatus is problematic in that it comprises a plurality of gas diffusing nozzles of different heights which will distribute the process gas to different levels irrespective of the location of wafers in the boat. The use of such an apparatus can give rise to variations in the density of the gas in the tube and an irregular chemical vapor deposition rate. Furthermore, the apparatus, which relies on a plurality of gas diffusing nozzles, has a correspondingly complex structure.
One proposal to solve the aforementioned problems, as shown in FIG. 2, is to employ only one gas diffusing nozzle 20 having a plurality of gas diffusing openings 21. However, this proposal still has problems. The quantity of gas which diffuses through the openings varies according to the height at which the gas diffusing opening is located. That is, the further up the gas diffusing opening is located in the nozzle, the smaller is the quantity of gas which will diffuse through the opening during a given time of operation. The density of the gas filling the internal tube decreases in a direction toward the top of the tube, and accordingly, the rate at which layers are formed on the surface of the wafers varies amongst the wafers situated in the boat. Hence, the resultant layers on the wafers do not have the same thickness.